Optical and magnetooptical data storage systems combine very high storage density with low cost, random access, and erasability. The commercial success of these systems has led to a technological push to further increase the areal density, data rates, access times and the erasability of these types of drives.
The basic principles of magnetooptical recording involves pulsing a focused laser beam to a high power for a short period of time in order to raise the temperature of a perpendicularly magnetized medium. The temperature of the magnetized medium is elevated for a sufficiently long time such that an externally applied magnetic field can be used to reverse the direction of magnetization in the heated region. When the laser power is reduced, the active layer of the optical medium cools to a lower temperature. At a reduced temperature, the region of reversed magnetization persists.
To read the medium, the same laser beam is employed to sense the orientation of the magnetized regions previously encoded. Information is then read from the optical disk in conformance with the optical Kerr effect. According to the optical Kerr effect, light reflected off the surface of the magnetic material is optically polarized. The polarization angle of the electromagnetic radiation reflected from the surface of the magnetic material is a function of its magnetization. This polarization is the result of the interaction between the photons and the molecular order imposed by the magnetizing field. The stronger the magnetic field the greater the molecular ordering, and the more that the ordering influences the incident radiation.
Optical recording systems take advantage of the polar Kerr effect during read-out of information by sensing in which direction the linearly polarized light reflected from a perpendicularly magnetized medium will be rotated (e.g., to the left or right according to the direction of magnetization). Thus, by simply checking the direction of the plane of polarization of a reflected light, information can be read using the same laser that was utilized to write the information to the medium. (It is important to note that reading has no influence on the state of magnetization since the read laser power is relatively low. Hence, read-out produces an insufficient temperature rise to alter the direction of magnetization within the optical medium).
One of the problems that arises in optical disk drive systems is the presence of contaminated particles or defects on the surface of the optical disk. Investigation into the effect of dust and dirt particles on the surface of the optical disk indicates that the most serious impact is on the write process. The read-back process has been demonstrated to be relatively immune to this phenomena.
The crux of the problem lies in the fact that a dust particle is capable of blocking a portion of the incoming laser light during write operations. This light, as previously discussed, is used to raise the temperature of the magetized medium. However, in blocking a portion of the incoming light the dust particle effectively reduces the available write power being delivered to the magnetizing layer at the very point where the data is to be recorded. If the write power drops below a certain threshold level, the underlying perpendicularly magnetized medium fails to reach a high enough temperature capable of reversing the direction of magnetization.
The ramification, of course, is that the dust particle (or other contaminate particle) prevents the data from being properly recorded. In this respect it has been observed that even a very small dust particle on the disk surface can adversely affect data integrity over an area of more than five times the diameter of the particle itself. As a consequence, uncorrectable errors are caused, even in disk drive systems incorporating extensive ECC correctional capabilities.
What is needed then is a way to compensate for the reduction in power at the active surface of the medium due to the presence of a dust particle on the surface of the disk. As will be seen, the present invention discloses a novel circuit which is capable of detecting the presence of a contaminate particle on the optical disk drive's surface. The invention then increases the write power of the laser over the area of the disk where the contaminate particle is blocking the incoming laser light. The increase in laser power is made to be proportional to the reduction sensed. The additional laser power overcomes the loss of light phenomena. Consequently, drive systems incorporating the present invention are capable of writing data reliably--even in the presence of very large (i.e., up to 0.5 millimeters) dust particles.